Range Request File Server

package main

import (
	"fmt"
	"io"
	"net/http"
	"net/http/httptest"
	"os"
	"path"
	"path/filepath"
	"strings"
)

func fileHandler(root string) http.HandlerFunc {
	absoluteRoot, err := filepath.Abs(root)
	if err != nil {
		panic(err)
	}
	return func(w http.ResponseWriter, r *http.Request) {
		cleaned := path.Clean("/" + r.URL.Path)
		full := filepath.Join(absoluteRoot, filepath.FromSlash(strings.TrimPrefix(cleaned, "/")))
		if !strings.HasPrefix(full, absoluteRoot+string(os.PathSeparator)) {
			http.Error(w, "invalid path", http.StatusBadRequest)
			return
		}
		file, err := os.Open(full)
		if err != nil {
			http.Error(w, "not found", http.StatusNotFound)
			return
		}
		defer file.Close()
		info, err := file.Stat()
		if err != nil || info.IsDir() {
			http.Error(w, "not found", http.StatusNotFound)
			return
		}
		http.ServeContent(w, r, info.Name(), info.ModTime(), file)
	}
}

func main() {
	root, err := os.MkdirTemp("", "range-server-*")
	if err != nil {
		panic(err)
	}
	defer os.RemoveAll(root)
	if err := os.WriteFile(filepath.Join(root, "report.txt"), []byte("hello range world"), 0o644); err != nil {
		panic(err)
	}

	server := httptest.NewServer(fileHandler(root))
	defer server.Close()

	request, err := http.NewRequest(http.MethodGet, server.URL+"/report.txt", nil)
	if err != nil {
		panic(err)
	}
	request.Header.Set("Range", "bytes=6-10")
	response, err := http.DefaultClient.Do(request)
	if err != nil {
		panic(err)
	}
	defer response.Body.Close()

	body, err := io.ReadAll(response.Body)
	if err != nil {
		panic(err)
	}
	fmt.Println("status:", response.StatusCode)
	fmt.Println("content-range:", response.Header.Get("Content-Range"))
	fmt.Println("body:", string(body))
}